In the use of air and hydraulic cylinders, it has been found that magnetic switches typically magnetic reed switches may be substituted for mechanical limit switches. The reed switch opens and closes in response magnetic flux. This flux is generally produced by a permanent magnet which is attached to a piston within the cylinder body. Movement of the piston, and consequently the magnet, varies the magnetic flux and opens and closes the associated reed switch. Limit switches of this type are in widespread use and have an almost infinite number of potential applications. They may be used for automatic cylinder cycling, light indication, cylinder programming and sequencing, grip timers and counters, multi-position signalling and countless other applications. Their use frequently permits the elimination of auxiliary gears, spiral rod extensions, switch dogs, mounting plates, cams as well as many other elements.
Prior art devices have attached these magnetic switches to tie rods juxtaposed to the cylinder body. These devices, however, experience failures. One type of failure results from high speed operations of the piston in which the resonance time of the requisite level of magnetic flux is insufficient to activate the magnetic switch. This problem is compounded as the cylinder is subjected to various mechanical vibrations. These vibrations tend to cause relative rotation between the magnetic switch and the tie rod to which it is secured. This relative rotation increases the air gap between the switch and a magnet and diminishes the magnitude of magnetic flux experienced by the switch as the magnet passes. The prior art devices have used tie rods of circular cross section and relied upon frictional forces exerted upon a tie rod positioned between the switch housing and an associated fastening plate to prevent relative rotation. Some prior art devices have even provided serrations upon the fastening plate. The result however has been unsatisfactory. The applicant has found that substantial improvement in reliability results from further reducing the probability of relative rotation between the tie rod and the magnetic switch and minimizing the air gap between the magnet and switch. It has further been found that substantial reductions in the probability of relative rotation between the switch and tie rod may be had by cooperating mated flat surface portions on the tie rod and switch or by tightly engaging the switch at a location in which the diametral dimension is at a relative minimum.
It shall therefore be an object of this invention to provide a more reliable magnetic switch.
It is a further object of this invention to provide a positive means on the cylinder tie rod to prevent relative rotation with an attached magnetic switch.